The present invention relates to an automatic detection system for use in materials-conveyance systems in which a series of items are fed onto a moving surface at uniform intervals and conveyed on said moving surface for individual treatment in one or more stations, the detection system being concerned with automatically-sensing changes in the normal conveyance time for each of said items and producing a signal when predetermined abnormal changes occur in said normal conveyance time.
Primarily, the present invention relates to a paper handling machine, such as an electrostatic copying machine in which a succession of paper copy sheets are moved individually and in spaced relation against a moving continuous photoconductor web and through a series of work stations including a transfer station and a heat-treatment or fuser station in which the toner images transferred to each copy sheet are heated to an elevated temperature to fuse the toner powder to the copy sheet as the copy sheet passes therethrough.
As can be readily understood, a fire hazard is created if one of the paper sheets becomes trapped or lodged within the fuser station since the heater remains energized while the copy sheet is in the fuser station. Unless detected very quickly, successive copy sheets continue to be moved into the fuser station, resulting in a jam, and possible ignition of said copy sheets.
In realization of this problem, prior-known electrostatic copying machines have been provided with jam-detection systems which use photocells to sense the movement of articles on a conveyor surface and to load a shift register to provide a signal when the article should have moved a predetermined distance. The clock pulses to the shift register originate with a clock pulse generator which provides a series of pulses of uniform frequency independently of the rate of movement of the conveyor surface. The signal from the stage register and signals from a number of bistable devices or gates and associated photocells cooperate to deactivate the copying machine if such time period exceeds a predetermined time lapse during which the sheet would normally pass through the various stations of the machine. Thereafter, the operator must open the machine to clear any jammed paper from the fuser station and reset the machine for re-operation. Reference is made to U.S. Pat. No. 3,778,629.
While such prior-known systems are effective for their intended purpose, they have certain disadvantages. The procedure of opening the machine and clearing any trapped paper from the fuser station is time-consuming and frequently requires a trained technician or operator. In some instances, the machine must be left idle and the copying procedure left incompleted until such a trained person can attend to the clearing operation. This difficulty cannot be avoided whether a true jam has occurred or whether the machine has sensed a jam where no jam exists, in fact. This latter situation can occur in certain cases, such as where a voltage change may cause the speed of the conveyor surface to vary from normal, or where the paper movement may be hindered for an instant due to slippage, or other cause. In such cases, there is no trapping of the paper within the fuser and no fire hazard. However, the logic system of the machine senses an irregularity in the time delay for the paper feed and deactivates the machine so that it cannot be re-operated without opening the fuser station and resetting the machine.
Prior-known systems also employ photocells to detect the movement of each copy sheet along its intended path. Such photocells are subject to erroneous activation by paper scraps or other foreign matter which may collect on the conveyor belt.
Prior-known systems are also in use which are based upon the detection of spacing or gaps between articles being conveyed. Such systems are unsatisfactory because they are too slow to react to certain problems, such as where the leading edge of a sheet becomes jammed. The machine logic cannot signal such a jam until after a period of time exceeding the period during which the entrance switch would normally be energized.